Investigation of Carrier Dynamics in Templated Perovskite Films with Different Densities of Nanopores

Abstract

Carrier dynamics in templated perovskite films with different densities of nanopores were investigated. We employed colloidal solution processing to obtain templated perovskite films with different number densities of nanopores. X-ray diffraction (XRD) and photoluminescence (PL) measurements confirmed that porous structures dominantly influence a fast carrier decay process such as carrier extraction, while PbI 2 residuals influence a slow carrier decay process.Keywords: Perovskite solar cell | Nanopore density | PbI 2 residualRecently, organo lead halide perovskite CH 3 NH 3 PbX 3 has emerged as a promising candidate for low-cost and highefficiency photovoltaic technologies. 17 Up to now, power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been dramatically improved by employing planar structures, with a dense and smooth perovskite film. On the other hand, perovskite films with porous structures have stimulated great interest since some researchers obtained semi-transparent perovskite films with high optical transmittance 8,9 and periodic change of refractive index, 10,11 which are favorable to apply to various optoelectronic devices such as solar cells, light emitting diodes (LEDs), and lasers. In previous works, tremendous efforts have been devoted to obtaining devices with preferable performance. However, the fundamental carrier dynamics such as carrier extraction and radiative recombination have not been comprehensively discussed. For example, in a solar cell configuration, it was reported that the electron extraction from a perovskite layer to an electron-transporting layer (ETL) was suppressed by poor surface coverage of TiO 2 by perovskite.12 However, high PCE was exclusively aimed and the impact of surface coverage on electron extraction was not intensively investigated. For the purpose of designing optoelectronic devices with porous structures, the correlation between the number densities of nanopores and carrier dynamics needs to be systematically investigated.Herein, we employed polystyrene (PS) nanospheres, which are commonly used in templating techniques, to fabricate templated perovskite films with porous structures as illustrated in Figure 1a. The impact of porous structures on carrier extraction was investigated by photoluminescence (PL) measurement in a solar cell configuration with a TiO 2 compact layer as an ETL, where templated perovskite films with different number densities of nanopores were fabricated by changing the spinning speeds of PS colloidal solution.The fabrication process of the templated perovskite films is illustrated in Figure 1b. For the fabrication of templated perovskite, we firstly fabricated porous PbI 2 films using PS nanospheres and then perovskite (CH 3 NH 3 PbI 3 ) was formed based on a successive spin-coating/annealing (SSCA) method. 13The detailed experimental information can be found in Supporting Information.Firstly, we observed the surface morphology of PS films deposited by various spinning speeds. Figures 2a2c show the SEM images of the PS fil...